This invention relates to autoclaves.
Autoclaves are used for sterilization or other treatment of articles at elevated temperature in the presence of liquid vapor at elevated pressure. The articles to be treated are placed in a pressure vessel which includes a heater and a reservoir of liquid such as distilled water. In operation, the vessel is closed and the heater turned on so that the water is heated. The steam produced is allowed to vent from the vessel for a short time so as to flush air from the apparatus. The air vent is then closed so that pressure and temperature within the vessel increase to predefined levels. The atmosphere of high humidity and temperature in the vessel is sufficient to sterilize the articles after a predetermined time. When the sterilization cycle is complete, the heater is turned off and the vessel is allowed to cool, or is actively cooled such as by a fan. When the pressure in the vessel has dropped to a safe level, the door of the vessel is unlocked allowing the articles to be removed and other articles to be sterilized. Each cycle results in a loss of some of the water or other liquid in the vessel which must be replaced. This is generally achieved automatically by supply from a tank outside the pressure vessel at the start of a new cycle. A level sensor in the vessel reservoir detects when there is sufficient liquid present so as to prevent overfilling. Overfilling can be dangerous since, in some cases, it could result in hot water flooding out of the vessel door when this is opened at the end of the cycle.
Because of the adverse environment within the pressure vessel caused by the high pressure and temperature, the liquid level sensor used must be of a robust construction. Generally, the only form of sensor suitable for such an application is a conductivity sensor which detects the presence of a liquid by its effect on reducing the electrical resistance between two electrodes. One problem, however, with this form of sensor is that liquid may cling to it even when not actually immersed in the liquid so that it gives a false indication of the presence of liquid.